Alternator Problem

[samdsalk]

Just bought a 1980 CB750C.  When I got it i would have to jump it in order to start it (it would attempt to turn over).  It would run for about 30 mins then die.  I picked up a new battery and the bike started right up.  I rode for a bout 15 mins and i shut it off.  It started back up again.  I rode for about an hour and then shut it off again.  When i went to start it again it did not even try to turn over.  Then i tried a few mins later and it attempted to turn over.  Any ideas?
Thanks,
Sam

[Re-run]

Well you either have charging issues or a bad battery. Yes a new battery can be bad.
Charge the battery fully and take a voltage reading. Let it sit overnight and check again. If the voltage is about the same(.01-.03 or so) then battery should be good. hook the batt up and start the bike. rev to 2000 rpm and check voltage. It should be at least 13V minimum. If it is not, then from there it is time to dig into the system.